One proposed heat exchanger is an in-vehicle corrugated fin tube heat exchanger including multiple flat tubes arranged to make a coolant flow and corrugated fins attached between adjacent pairs of the multiple flat tubes (see, for example, Japanese Patent Laid-Open No. 2001-167782). One proposed structure of a cross fin tube heat exchanger uses multiple slit fins with thin slits formed therein (see, for example, Japanese Patent Laid-Open No. 2003-161588). Another proposed structure of the cross fin tube heat exchanger uses wavy fins with wave crests and wave troughs formed in a direction perpendicular to the direction of the air flow (see, for example, Japanese Patent Laid-Open No. 2000-193389). Still another proposed structure of the cross fin tube heat exchanger uses V-shaped wavy fins having wave crests and wave troughs arranged in a V shape at an angle of 30 degrees relative to the direction of the air flow (for example, Japanese Patent Laid-Open No. H01-219497). These proposed techniques adopt various shapes of fins with the purpose of accelerating heat transfer in the fin tube heat exchangers.